An amplifier device or buffer can require very high input impedance. For example, an amplifier device or buffer may have an input that is high ohmic when receiving signals from a capacitive sensor having a capacitance of a few picofarads. In some instances, a bias voltage may be needed to define the DC operating point of the amplifier device or buffer. A high impedance device, such as a device having an impedance in the giga-ohm range, may be placed between the amplifier device or buffer and a bias point to provide a high impedance input to the amplifier device or buffer. The high impedance device may utilize impedance transformations to provide a high impedance to the input of the amplifier device or buffer.
One impedance transformation technique relates to coupling a pair of transistor circuits in a current mirror type arrangement with each transistor circuit of the pair having a different channel width to channel length (W/L) ratio. In some implementations, the transistor circuit having a lower W/L ratio acts as an impedance device. In these implementations, the quotient in the W/L ratios of the transistor circuits is proportional to the impedance transformation. To illustrate, when the W/L ratio of a first transistor circuit is approximately ten times larger than the W/L ratio of a second transistor circuit, the impedance value of the second transistor circuit increases by a factor of ten.
Additionally, an impedance transformation may take place due to differences in overdrive voltages of transistor circuit pairs coupled in a current mirror type arrangement. An overdrive voltage is defined by a gate-source voltage of a transistor circuit minus a threshold voltage of the transistor circuit. A transistor circuit having an overdrive voltage lower than the overdrive voltage of another transistor circuit may serve as an impedance device. In some implementations, at least the transistor circuit with the lower overdrive voltage is operating with a gate-source voltage in the sub-threshold region. A transistor circuit is operating with a gate-source voltage in the sub-threshold region when the gate-source voltage is less than the voltage that turns on the transistor circuit (i.e. when the overdrive voltage is a negative value). When the gate-source voltage of the transistor circuit serving as the impedance device is in the sub-threshold region, the impedance increases exponentially in relation to the difference between the overdrive voltages of a pair of transistors. For example, when the overdrive voltage of the impedance device transistor is a specified amount lower than the overdrive voltage of another transistor circuit, the impedance of the impedance device transistor may increase by a factor of ten. In one illustration, the impedance of the impedance device transistor may increase by a factor of ten for every 80-90 mV difference between the overdrive voltages of the transistor circuits. In some implementations, the differences in the overdrive voltages may be produced via the gates of the transistor circuits. In other implementations, the differences in the overdrive voltages may be produced via the sources of the transistor circuits.
In some instances, transistor circuit pairs arranged in current mirror type arrangements to produce impedance transformations may have non-linear outputs. Further, even order distortion may produce rectifying effects that shift the DC operating point of the amplifier device or buffer.